The present invention pertains to the field of controls for cooling systems employing air-cooled condensers exposed to outdoor ambient temperatures. When such systems were intitially developed many years ago, they could not be used for year round outdoor service in colder climates, because the extremely low head pressures developed during the winter months would result in extremely poor efficiency or even complete inoperation of the system. Consequently, a number of different control techniques were developed to raise the head pressure in the condenser when it is exposed to cold ambient air, so as to maintain pressures as high as normally existing during summer months. Some such techniques involve the use of thermostatically controlled fans or dampers on the condenser to decrease its effectiveness during winter months. Another widely used technique is to provide a bypass path around the condenser for hot gas so that it may pass directly from the compressor to a receiver, thereby causing refrigerant to back up into, or flood, a portion of the condenser. In such systems, a control valve controls system pressure against a reference pressure, and controls flooding accordingly. When a portion of the condenser is flooded, the effective area of the condenser is reduced so that higher pressures can be maintained.
Although these types of prior art systems have proved to be generally satisfactory, further improvements are possible in terms of improving the energy efficiency of the system. Increased volumetric efficiency and decreased power consumption could be achieved if the system could be operated at a lower head pressure rather than maintaining summer head pressure year round. As condensing pressure drops, the compression ratio that the compressor operates against decreases. As the compression ratio decreases, the volumetric efficiency of the compressor increases. Volumetric efficiency is defined as how much a compressor actually pumps as compared to how much it would pump if there were no re-expansion or other losses. As the volumetric efficiency increases, the refrigerating effect increases in direct proportion. Also, as condensing pressure is lowered, the power consumption of the compressor generally declines. The theoretical net effect then, of operating at lower head pressures is an increase in refrigerating effect, accompanied in most cases by a decrease in power consumption by the compressor.
Unfortunately, a number of unwanted factors affecting system performance begin to show up at lower head pressures which, if uncontrolled, can disrupt system operation and cancel out the theoretic advantages of operating at low head pressures as discussed above. Of these factors, the formation of flash gas entering the expansion device, is the most serious. In most systems, the expansion device, often a thermostatic expansion valve, has a port designed to operate with liquid phase refrigerant. The presence of vapor bubbles or flash gas in the liquid refrigerant delivered to the expansion device can greatly disrupt its operation by in effect clogging the orifice and reducing its flow capacity.
It has been found that a number of more or less independent factors contribute to the formation of flash gas in the liquid line, and not all of these factors are predictable or easily controllable. As a result, prior art systems have had to set the reference pressure for their condenser flooding or other pressure control system high enough to avoid flash gas formation during the worst case combination of these factors. The result is that when the factors which contribute to flash gas formation are at less than a worst case condition, which might be 90% of the time, the system is operating at a higher pressure than necessary, resulting in operation at less than optimum efficiency.
A number of the factors which contribute to the formation of flash gas in the liquid line are as follows. The length of the liquid line may be a prime contributor. In many installations such as supermarkets, the condensers for cool space such as display cases or meat lockers may be operated a great distance away, for example on the roof of a building. The liquid line may run across a part of the roof, through a distance inside the comfort conditioned space of the building, and in some cases even through a non-air-conditioned equipment area. In addition to the pressure drop due to fluid resistance in the long fluid line, the long run provides an increased area for possible heat pick up when passing through areas at higher temperature than the liquid, or when exposed to the sun on the roof. Another problem results when the evaporator is positioned above the condensing unit, resulting in loss of liquid line pressure due to the static head. Also, either oversized or undersized liquid lines can contribute to flash gas generation. If the liquid lines are undersized, excessive fluid pressure drop is introduced because the liquid has to travel at a higher velocity to achieve the desired flow rate. On the other hand, if the liquid lines are oversized, they present an increased area or surface for heat transfer into the liquid line from higher ambient temperatures. Further, the reduced velocity of teh refrigerant within the oversized liquid line results in correspondingly greater time during which the refrigerant is exposed to the unwanted heat transfer.
It is common to provide subcooling to the refrigerant after it leaves the condenser and receiver. This is accomplished by routing the liquid line from the receiver through a number of coils mounted with the condenser. Since the refrigerant in the receiver normally is at saturation temperature and pressure, the subcooler will bring the refrigerant a few degrees below saturation temperature at that pressure, so as to provide a reserve or margin against the formation of gas bubbles. However, the temperature increases due to heat pickup mentioned above and pressure drops as mentioned above both tend to reduce or eliminate the subcooling margin by the time the refrigerant reaches the evaporator, so that the addition of a subcooler alone does not fully solve the flash gas problem.
Another type of prior art system attempts to solve this problem by passing the liquid line through a heat exchanger between the liquid line input to the evaporator and the gas line output from the evaporator. However heat exchangers rely on the entering liquid being warmer than the entering gas. This relationship cannot always be maintained during winter operation, and when it is not maintained, the heat exchanger is a liability, sending liquid to the compressor and flash gas to the expansion valve.